Potential Mechanisms Involved in the Negative Coupling Between Serotonin 5‐HT 1A Receptors and Carbachol‐Stimulated Phosphoinositide Turnover in the Rat Hippocampus

Serotonin 5‐HT 1A receptors have been reported to be negatively coupled to muscarinic receptor‐stimulated phosphoinositide turnover in the rat hippocampus. In the present study, we have investigated further the pharmacological specificity of this negative control and attempted to elucidate the mechanism whereby 5‐HT 1A receptor activation inhibits the carbachol‐stimulated phosphoinositide response in immature or adult rat hippocampal slices. Various 5‐HT 1A receptor agonists were found to inhibit carbachol (10 μ M )‐stimulated formation of total inositol phosphates in immature rat hippocampal slices with the following rank order of potency (IC 50 values in n M ): 8‐hydroxy‐2‐(di‐ n ‐propylamino)tetralin (8‐OH‐DPAT) (11) > ipsapirone (20) > gepirone (120) > RU 24969 (140) > buspirone (560) > 1‐( m ‐trifluoromethylphenyl)piperazine (1,500) > methysergide (5,644); selective 5‐HT 1B , 5‐HT 2 , and 5‐HT 3 receptor agonists were inactive. The potency of the 5‐HT 1A receptor agonists investigated as inhibitors of the carbachol response was well correlated ( r = 0.92) with their potency as inhibitors of the forskolin‐stimulated adenylate cyclase in guinea pig hippocampal membranes. 8‐OH‐DPAT (10 μ M ) fully inhibited the carbachol‐stimulated formation of inositol di‐, tris‐, and tetrakisphosphate but only partially antagonized (‐40%) inositol monophosphate production. The effect of 8‐OH‐DPAT on carbachol‐stimulated phosphoinositide turnover was not prevented by addition of tetrodotoxin (1 μ M ), by prior destruction of serotonergic afferents, by experimental manipulations causing an increase in cyclic AMP levels (addition of 10 μ M forskolin), or by changes in membrane potential (increase in K + concentration or addition of tetraethylammonium). Prior intrahippocampal injection of pertussis toxin also failed to alter the ability of 8‐OH‐DPAT to inhibit the carbachol response. Carbachol‐stimulated phosphoinositide turnover in immature rat hippocampal slices was inhibited by the protein kinase C activators phorbol 12‐myristate 13‐acetate (10 ü M ) and arachidonic acid (100 μ M ). Moreover, the inhibitory effect of 8‐OH‐DPAT on the carbachol response was blocked by 10 μ M quinacrine (a phospholipase A 2 inhibitor) but not by BW 755C (100 μ M ), a cyclooxygenase and lipoxygenase inhibitor. These results collectively suggest that 5‐HT 1A receptor activation inhibits carbachol‐stimulated phosphoinositide turnover by stimulating a phospholipase A 2 coupled to 5‐HT 1A receptors, leading to arachidonic acid release. Arachidonic acid could in turn activate a γ‐protein kinase C with as a consequence an inhibition of carbachol‐stimulated phosphoinositide turnover. This inhibition may be the consequence of a phospholipase C phosphorylation and/or a direct effect on the muscarinic receptor. The latter possibility is supported by the fact that incubation of immature rat hippocampal slices with 8‐OH‐DPAT (10 μ M ) caused a down‐regulation (‐16%) of [ 3 H]quinuclidinyl benzylate binding.